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This question is about light emission, which may overlap with physics, but I am most interested in combustion and types of flame (incandescence, petrochemical fuel flames, nuclear ractions).

Consider this article about candle flames. In it they describe how a candle flame only emits light where it has contact with oxygen (emphasis mine):

However the reaction can only occur where the air meets the wax vapour. This cannot happen all the way though the flame, just around the outside. So all flames are hollow.

How accurate is this? What parts of a flame emit light?

If flames have complex scattering and absorption properties distinct from the burning substance, does this only apply to the outside regions, with the interior being goverened by whatever fuel is burning?

I suppose "fire" is many different phenomena, so are the following flames also hollow?

A: Campfire

B: Rocket exhaust

C: Oil platform explosion

D: The Sun

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    $\begingroup$ The Sun is not hollow in any sense. Best not to extrapolate from a candle flame. $\endgroup$ – Ed V Apr 9 at 12:21
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    $\begingroup$ Emission does not need oxygen at all. It needs high temperature, that was achieved by previous oxidation by oxygen. In fact, it is the opposite - as the oxygen content decreases by reacting, emission increases, as the temperature is higher. $\endgroup$ – Poutnik Apr 9 at 12:25
  • $\begingroup$ Does this mean that the article about candle flames is inaccurate? They demonstrate the effect. $\endgroup$ – dubious Apr 9 at 12:29
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    $\begingroup$ Candle flames glow because they are sooty. Incomplete combustion leads to soot, which glows due to black body radiation from the heat. This can only happen in the outer part of the flame where there is oxygen because you need some combustion. Much of the inner part is vaporized tallow and does not have a high enough oxygen content to burn. This only applies to small-ish flames that are fed via atmospheric oxygen. Compare with a Bunsen burner, which does premixing, and the flame does not glow because combustion is nearly complete. $\endgroup$ – Zhe Apr 9 at 12:36
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    $\begingroup$ Flames are not all the same. The burning gases in a bunsen-burner flame (or a gas stove) are premixed with air enabling combustion throughout the flame. Candle flames don't do that and a re bad basis for judging other flames. $\endgroup$ – matt_black Apr 9 at 12:58
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The answer is NO.

The article you quote makes a completely unwarranted generalisation that "all flames are hollow". This is true of some flames but only because the fuel that is burning is only able to burn when mixed with oxygen from the air. In those flames, the flame is "hollow" because only in regions where air can mix with the fuel can the fuel burn and the rate of mixing is dependent on the flow of fuel and the rate at which air can mix or diffuse into the fuel rich region. Bunsen burner flames are like this when the air is not premixed with the fuel gas (because the air intake on the bae is closed) and candle flames are like this because the wax has to evaporate to mix with air.

A bunsen flame when the air intake is open is not like that as the fuel gas and air are pre-mixed and can burn throughout most of the region of the flame.

Another factor worth considering is what causes the emission of light. In candles and many other poorly controlled flames, there is a lot of light from incandescence not from the underlying burning reaction. Incandescence involves black-body emissions from hot solid particles. Those particles are usually small particles of solid carbon produced from partial, incomplete burning of hydrocarbons. They emit light because they are hot. Candles are tuned to create a lot of these particles as this gives far more light than complete combustion. In contrast, a well-controlled bunsen flame emits very little light (the light is not black-body emission but comes from specific excited states in the reactants).

As to how this applies to your specific examples here are some analyses.

1. Campfires

Campfires are a mixture of various phenomena. They have flames a little like candle flames. But they also have very hot solids which emit a lot of black-body radiation because they are hot. Some of the emitted gasses have little emission of light as they are like a bunsen flame, but the overall effect is dominated by the hot solids. And, of course, some camp fires have far better mixing of air and fuel than others so it is hard to generalise.

2. Rocket exhaust

Most rockets need to be as efficient as possible. So they tend to have very carefully designed mixing of fuel and oxidiser to achieve maximal energy output. They are not hollow and tend not to emit huge amounts of light (compared to the equivalent candle flame anyway).

3. Oil platform explosion

Impossible to generalise. But unintended explosions don't tend to be well controlled so will have some areas where mixing is good and others where it is not so some areas are like a candle and others like a bunsen. Incidentally, movie explosions emit a lot more light than real explosions because, well, hollywood likes visual spectacle not realism (see this youtube explanation and illustration of how movies deliberately add fuel to create poorly mixed candle like flames for more visually interesting explosions).

4. The sun

The sun does not emit light because of burning (at least not in the normal chemical sense). The sun emits light because the surface is very hot. And this happens because of nuclear fusion deep in its core (not what a chemist would call burning, though a nuclear physicist might, but what do they know?). The light we see is mostly black body emissions because the surface is hot. It is certainly not hollow either.

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    $\begingroup$ To add to this excellent answer, you might find it interesting that it takes photons originating from the core of the sun thousands of years to reach the sun's surface. $\endgroup$ – Paul Apr 9 at 17:21
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    $\begingroup$ The light emission from rocket exhaust depends very much on what kind of fuel the rocket is burning. The old NASA Space Shuttle provided a very nice contrast of extremes: the main engines on the shuttle itself burned pure hydrogen and oxygen and had flames that were nearly invisible; the solid-fuel rocket boosters on the sides, meanwhile, burned a mixture of ammonium perchlorate, plastic binder and powdered aluminum (of all things!) and had exhaust plumes that were incandescent as heck. In pictures and videos of shuttle launches you can see both engines firing at the same time. $\endgroup$ – Ilmari Karonen Apr 10 at 13:47
  • $\begingroup$ @IlmariKaronen Good point. I am also guilty of overgeneralisation! $\endgroup$ – matt_black Apr 10 at 15:53

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